For inhibiting corrosion of the inner and outer surfaces of metal cans, paints are conventionally applied, wherein used as the paints are thermosetting resins.
Other methods include the use of thermoplastic resins. For example, a polyolefin film is laminated with a tin-free steel heated or a polyester film with good heat resistance is laminated with a metal sheet to be used for cans.
Many thermosetting paints are of solvent type. For forming a coating therefrom, a high temperature heating for a long time at 150.degree.-250.degree. C. for several minutes and baking which undesirably causes evaporation of a large amount of organic solvent are necessary. Accordingly, simplification of steps and improvements to prevent pollution are desired. It is inevitable, moreover, that a small amount of organic solvent remains in the coating when the thermosetting paints are applied under the above-mentioned conditions and the remainder of the organic solvent migrates into the food packed in a metal can applied with the coating to result in degraded taste and smell of the food. Also, the additives in the paint or low molecular substances which may be produced by an incomplete crosslinking reaction may move into the food to exert the same adverse influences.
Of the above-mentioned problems, simplification of the steps and prevention of pollution can be achieved by the use of thermoplastic resin films. However, polyolefin films such as those made from polyethylene and polypropylene, which are among thermoplastic resins, have poor heat resistance and sometimes whiten and peel off from a metal sheet upon a retort treatment. In addition, polyolefin films are soft and poor in scratch resistance. The poor scratch resistance of the film causes problems that, for example, scratches occur on the film surface during transporting each laminated metal sheet or seaming sheets into a can in a can manufacture process, thus impairing the product value.
While the problem caused by the migration of residual solvent as observed when a thermosetting paint is applied can be overcome by the use of polyolefin films, the migration of low molecular substances produced during forming a coating or that of additives such as a heat stabilizer into food still exerts disadvantageous effects on taste and smell of the food. The polyolefin film, moreover, adsorbs aromatic ingredients in the food and is poor in flavor resistance.
On the other hand, the use of a polyester film from among the thermoplastic resin films is most preferable because the above-mentioned difficulties which polyolefin films encounter can be overcome.
Although polyester films generally show superior scratch resistance as compared with polyolefin films, the resistance is not entirely satisfactory and an improvement thereof is desired. So as to improve the scratch resistance, the application of an organic coating superior in lubricating property and scratch resistance to the surface of a polyester film has been proposed. While the scratch resistance can be unquestionably improved by this coating method, this method necessitates the use of an organic solvent during the process of forming an organic coating and a very small portion of the solvent remains in the organic coating obtained. Accordingly, when a food is packed in a metal container manufactured from this film, the organic solvent adversely affects the taste and smell of the food. There is also a problem that low molecular substances elute out from the organic coating to exert adverse influence as does the residual organic solvent as described.
The polyester film is superior in heat resistance so that an additive such as a heat stabilizer is not necessary and it produces less amounts of low molecular substances. As a result, the problems of degraded taste and smell of food caused by the migration of additives or low molecular substances can be markedly alleviated as compared with polyolefin films. The polyester films mainly comprising polyethylene terephthalate, however, contains low molecular substances produced during polymerization process and film forming process, i.e. oligomer based mainly on a cyclic ethylene terephthalate trimer (hereinafter also referred to merely as oligomer), and said oligomer may elute out from the film into food or may precipitate on the surface of a laminate film, giving rise to an appearance problem.